Error : Too many forms open are currently open in AX 2012 R3

Hello All,

I work in AX 2012 R3 and I have created a customization form and below are steps in this form:

1. There are three grids in customization form. All grids contain the "Service Order" details like --ServiceOrder, Customer Account, Project Id, Project Name etc...

2. After that , I click any record from grid in customization form.It will open the "Service Order" Form then I complete my relevant tasks in "Service Order" form.

4. Then close "Service Order" form to return the customization form.

But suddenly,  I face the following error : "Too many forms are currently open,close some forms and try again.

Can anyone suggest the solution? Thanks in advance